In the manufacture of semiconductor devices and the like, a stepping or step-and-scan projection exposure device (aligner) has been used in which a pattern of a reticle (photomask) is transferred onto each shot region on a wafer provided with a photoresist through a projection optical system.
The resolution of the projection optical system provided in the projection exposure device increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength which is the wavelength of radiation used in the projection exposure device has been reduced along with scaling down of integrated circuits, and the numerical aperture of the projection optical system has been increased.
The depth of focus is also important for exposure in addition to the resolution. The resolution R and the depth of focus δ are respectively shown by the following expressions.R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)
In the above expressions, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. When obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.
In the above case, a photoresist film is formed on the surface of the exposure target wafer, and the pattern is transferred to the photoresist film. In a related-art projection exposure device, the space in which the wafer is placed is filled with air or nitrogen. When the space between the wafer and the lens of the projection exposure device is filled with a medium having a refractive index of n, the resolution R and the depth of focus δ are shown by the following expressions.R=k1·(λ/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
For example, when using water as the above medium in the ArF process, since water has a refractive index n for light with a wavelength of 193 nm of 1.44, the resolution R is 69.4% (R=k1·(λ/1.44)/NA) and the depth of focus is 144% (δ=k2·1.44λ/NA2) of the values during exposure using air or nitrogen as the medium.
The above projection exposure method in which the wavelength of exposure radiation is reduced to transfer a more minute pattern is called an immersion exposure method. The immersion exposure method is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several ten nanometers (Japanese Patent Application Laid-open No. 11-176727).
In the immersion exposure method, both the photoresist film to be applied and to be formed on a wafer and the lens of the projection exposure device respectively contact with water. Therefore, water may permeate the photoresist film, whereby the resolution may be decreased. Moreover, the photoresist components may be eluted (dissolved) in water, thereby causing contamination of the lens surface of the projection optical system.
An upper layer film may be formed on the photoresist film in order to protect the photoresist film from water. The upper layer film which exhibits sufficient transparency for the exposure wavelength, can form a protective film on the photoresist film without being intermixed with the photoresist film, is not eluted into water during immersion exposure to maintain a stable film, and is easily dissolved in an alkaline solution as a developer are demanded.